1. Field of the Invention
The present invention relates to an image forming apparatus provided with a fixing unit for heating and melting unfixed toner and fixing it to a sheet bearing a toner image while permitting the sheet to pass a nip between a pair of rollers heated or between a heating belt and a roller.
2. Description of the Related Art
In recent years, attention has been focused on belt-type image forming apparatuses, in which a smaller heat capacity can be set, due to demands of shortening a warm-up time and saving energy in a fixing unit (see, for example, Japanese Unexamined Patent Publication No. H06-318001). Attention has been also focused on an electromagnetic induction heating method (IH) with a possibility of quick heating and high efficiency heating in recent years, and many products as a combination of electromagnetic induction heating and the employment of a belt have commercialized in light of saving energy upon fixing a color image. In the case of combining the employment of a belt and electromagnetic induction heating, an electromagnetic induction device is often arranged at an outer side of the belt due to merits that a coil can be easily laid out and cooled and further the belt can be directly heated (so-called external IH).
In the above electromagnetic induction heating method, various technologies have been developed to prevent an excessive temperature increase in a sheet non-passage area in consideration of a sheet width (paper width) passed through the fixing unit. Particularly, the following prior arts are known as size switching means in the external IH (for example, Japanese Unexamined Patent Publication No. 2003-107941, Publication of Japanese Patent No. 3527442).
A first prior art (Japanese Unexamined Patent Publication No. 2003-107941 (FIGS. 2 and 3)) discloses that a magnetic member is divided into a plurality of pieces, which are arranged in a sheet width direction, and some of the magnetic member pieces are moved toward or away from an exciting coil in accordance with the size of a sheet to be passed (paper width). In this case, heating efficiency decreases by moving the magnetic member pieces away from the exciting coil in sheet non-passage areas, and the amount of heat generation is thought to be less than in an area corresponding to a sheet with a minimum paper width.
A second prior art (Publication of Japanese Patent No. 3527442 (FIG. 10)) discloses that other conductive members are arranged outside a minimum paper width in a heating roller and the positions thereof are switched between those inside and outside the extent of a magnetic field. According to this prior art, the conductive members are first located outside the extent of the magnetic field to heat the heating roller by electromagnetic induction. If the temperature of the heating roller rises to the vicinity of a Curie temperature, the conductive members are moved to the extent of the magnetic field, thereby causing magnetic fluxes to leak from the heating roller outside the minimum paper width for the prevention of excessive temperature increases.
In the above first and second prior arts, an excessive temperature increase suppressing effect better than before is necessary to improve productivity more than at present. In other words, even if magnetism is shielded outside the paper passage area in the case of passing a small-size sheet, the temperatures of the heating roller and the like increase by the leaking magnetic field and eventually reach excessive temperature increases if the magnetic field (magnetic fluxes) leaks little by little from there. As means for preventing this, a method for making, for example, the area of a magnetic shielding plate larger than at present can be thought.
However, if the area of the shielding plate is excessively enlarged, the magnetic field is influenced even if the shielding plate is retracted. For example, even if the shielding plate is retracted when induction heating is necessary over the entire width of the heating roller, induction heating becomes insufficient, which is not suitable, if the magnetic field is shielded by the influence of the shielding plate at the retracted position. Therefore, there is a natural limit in enlarging the area of the shielding plate even if the excessive temperature increase suppressing effect is improved more than at present.